The gating on or closing of switches at the zero point in the current cycle of the current supplied between a source and a load is known in the prior art. Indeed, zero phase switching of multi-phase systems, particularly three phase systems, is also known in the prior art. The prior art, however, has had a great deal of difficulty in gating on or closing the load controlling switches at the substantially zero point in the current cycle.
The first difficulty is usually due to the fact that such zero phase switching circuits are designed for pure resistive loads; but in practice all resistive loads, and most of the main sources, have minimal inductance. This inductance results in a difference in the phase angles between the voltage and the current supplied by the source to the load through the switches. The second difficulty of typical prior art three phase switching systems is that they are synchronized to their respective line to neutral voltages which are in phase with the line currents during the on cycle. However, at the beginning of every on cycle, the switch, in three phase systems, which turns on first will see a line to line load only and its current will, therefore, be out of phase from the line to neutral voltage resulting in phase firing, rather than zero phase firing, and consequential radio frequency interference. The control problem when the voltage and current are out of phase is that the switches will be turned on or closed quite likely at a point in the current cycle other than the zero point which will result in the generation of radio frequency interference.
Furthermore, when solid state switches are used to control the supply of current from a source to a load, protective circuits must be devised to protect the solid state switches from damaging over voltages and dv/dt transients.